Catalytic reactions dehydrogenative coupling

In the last years transition metal-silyl complexes have received special attention for several reasons [1, 2], On the one hand, they are assumed to be important intermediates in catalytic processes [2] (transition metal-catalyzed hydrosilylation reaction, dehydrogenative coupling of silanes to polysilanes, etc.), on the other metal-substituted silanes show special properties, which can be tuned systematically by judicious choice of the metal and its ligands [3] Furthermore, silylenes (silanediyls) are stabilized by unsaturated transition metal fragments leading to metal-silicon double-bonds [4]. In the light of a possible application in MOCVD processes some of these complexes are of interest as potential single-source precursors for the manufacture of thin silicide films [5]. 

Hydride species were also formed in the dehydrogenative coupling of hydrosilanes with DMF [45]. The catalytic system is applicable to tertiary silanes, which are known to be difficult to be converted into disiloxanes (Fig. 17). The catalytic reaction pathway involves the intermediacy of a hydrido(disilyl)iron complex... 

In the course of studying the reactions of Si-H compounds with dialkyltitanocenes, with a view to the synthesis of new hydridosilyltitanocene complexes, we adventitiously discovered that phenylsilane undergoes facile, quantitative dehydrogenative coupling to a linear poly(phenylsilylene) under the catalytic influence of dimethyltitanocene. The ease with which this reaction proceeds initially induced us to underestimate the significance of the observation. 

The same catalytic system as described for the CDC of amines and ni-troalkanes, complemented with C0CI2 as a co-catalyst, also proved efficient for the allylic alkylation via cross-dehydrogenative coupling between various cycloalkenes and diketones (Eq. 9). Again, the exact mechanism or role of the organic peroxide are not known to date, but the formation of water probably provides the thermodynamic driving force for these reactions [121,122]. 

Catalytic dehydrogenative coupling of organohydrosilanes, a common method for the formation of linear polysilanes, can also be applied to cyclopolysilane synthesis. Thus, benzylsilane reacts to give all-fraws-hexabenzylcyclohexasilane in the presence of dimethyltitanocene (equation 3). However, extremely long reaction times are required and only moderate yields are obtained15. 

SiH-functional poly(silylated) aromatic hydrocarbons are important starting materials for the preparation of arene-bridged polysilanes through thermal or catalytic dehydrogenative coupling reactions. 

Further improvement in the reaction was possible by exclusion of the need for base. Thus, reaction of complex 7 with KO Bu resulted in the formation of the coordinatively unsaturated, 16e Ru(ll) neutral complex 8. Indeed, 8 is an excellent catalyst for the dehydrogenative coupling of alcohols to esters and the reaction proceeds with liberation of Hj under neutral conditions [llj. Table 1.2 provides a few examples. GC analysis of the reaction mixtures indicated that aldehydes were formed only in trace amounts. This catalytic reaction provided a new green pathway for the synthesis of esters directly from alcohols. Considerably less efficient methods had been reported previously for this transformation [34]. 

Summary Recent achievements in two catalytic reactions, i.e., silylative coupling and cross-metathesis of alkenes and dienes with vinyl-silicon compounds, which resulted in new synthetic routes to organosilicon molecular and macromolecular compounds are presented. The silylative coupling, also called dehydrogenative or trans-silylation and silyl group transfer, is catalyzed by metal complexes which either contain or initiate the formation of M-H and M-Si bonds, where M = Ru, Rh, Co and Ir. Cross-metathesis, which was developed very recently, proceeds in the presence of metallacarbenes, mainly those of rathenium (e.g., Grabbs catalyst). 

Two new catalytic reactions occurring between the same parent substances have been developed since 1984, i.e. the silylative coupling (dehydrogenative or trans-silylalion, silyl group transfer) and cross-metathesis of alkenes with vinylsilicon compounds. They make it possible to synthesize molecular and macromolecular compounds with vinylsilicon functionality. 

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